A reciprocating seal of this kind has been conventionally known that is used for a shock absorber mounted in an automobile and the like.
Among reciprocating seals according to a conventional art like this is, for example, a seal shown in FIG. 3.
FIG. 3 is a schematic sectional view of a reciprocating seal according to a conventional art.
The reciprocating seal is arranged in an annular space between a shaft (not shown) and the inner periphery of a housing (in more detail, shaft hole made in the housing), which move relatively in the direction of the shaft, to form a sealed space.
A reciprocating seal 100 according to the conventional art, as shown in the drawing, is mainly provided with a metal ring 200 and a rubber seal 300 baked on the metal ring 200.
The rubber seal 300 has an outer peripheral seal portion 301 mounted on the inner periphery of the housing, a main seal lip 302 brought into sliding contact with the outer peripheral surface of the shaft, and a sub-seal lip 303 similarly brought into sliding contact with the outer peripheral surface of the shaft.
The reciprocating seal constructed in this manner is required to improve frictional force characteristics.
That is, it is required to reduce fine vibrations caused when the reciprocating seal is brought into sliding contact with the surface of the shaft to a minimum.
This is because, for example, when the reciprocating seal is used for the shock absorber of an automobile, as the frictional force characteristics are greater, vibrations are reduced to improve the riding comfort of the automobile.
Then, to improve frictional force characteristics, as shown in FIG. 3, the main seal lip 302 has a two-step lip structure including the first lip 302a of the first step and the second lip 302b of the second step.
In this manner, both of the first lip 302a and the second lip 302b are brought into sliding contact with the surface of the shaft to stabilize the position of the main seal lip 302.
With this, the frictional force can be made uniform to improve frictional force characteristics.
However, even if the second lip 302b of the second step is brought into sliding contact with the surface of the shaft to make the frictional force uniform, the sliding contact of the second lip 302b increases the frictional force.
Hence, this prevents the smooth sliding contact of the main seal lip 302, and the frictional force characteristics can not be improved sufficiently.
Therefore, a technology of roughening the surface of the second lip of the second step to reduce frictional force by the second lip 302b has been developed (for example, see Japanese Unexamined Patent Publication No. 2001-355740).
Structures of roughening the surface of the second lip in this manner include structures of matting the surface of the lip, forming spiral screw protrusions on the surface of the lip, and forming parallel protrusions vertical to the shaft on the surface of the lip (see the above patent publication).
However, in the case of the structure of matting the surface of the lip, the surface of the lip is formed in random asperity.
For this reason, the amount of leak of sealed-in fluid (which is usually oil, so descriptions will be provided below, assuming that the sealed-in fluid is oil) becomes nonuniform in whole and hence there is a possibility that the amount of leak might increase in part.
Further, there is a possibility that also the frictional force might be nonuniform in whole.
In other words, in the case of the structure of matting the surface of the lip, there is presented a problem that it is difficult to control the amount of leak and the distribution of frictional force with accuracy.
Further, in the case of the structure of forming spiral screw protrusions or the structure of forming parallel protrusions vertical to the shaft, when the surface of the lip is brought into sliding contact with the surface of the shaft, these protrusions scrape off the oil.
Therefore, there is presented a problem that the thickness of an oil film is made too thick to increase the amount of leak of the oil.